The present invention relates to video displays, and more particularly to methods of characterizing video displays.
It is well known that the output luminance on a video display such as a CRT, liquid crystal, or light emitting diode display is a nonlinear function of the corresponding code value (or voltage), and a typical function 10 relating output luminance to input code value is shown in FIG. 1. This relationship is called the gamma curve. When the gamma curve is plotted in a logarithmic scale for both axes, the curve becomes more or less a straight line. The slope of this line is usually called the gamma value. It is often the case that the logxe2x80x94log version gamma curve is not strictly a straight line and the gamma value provides an imprecise description of the gamma curve. Therefore, more sophisticated relationships have often been used to model this curve. One such relationship is:
L(V)=xcex1(V+xcex2)xcex3,xe2x80x83xe2x80x83(1) 
where L is the output luminance of the display, V is an input code value used to indicate a desired luminance for the display, and xcex1, xcex2, and xcex3 are parameters to be determined from a best fit to a set of measured data.
This nonlinear relationship has a severe consequence on the luminance distribution of the displayed images. In a simple image display system the image intensity is converted linearly to the code value, which servers as input to the display. If the input to output relationship defined in Equation 1 for the display is not a linear function, the luminance distribution of the displayed image is distorted from the desired distribution. Therefore the image quality on a given display can be quite different from that of another display or hard copy output device such as an inkjet or laser printer.
There are two ways to avoid this problem. One is to convert the intensity of an original image to code values based on a nonlinear function that compensates for the gamma curve of the display. The other, which is more often used in psychophysical experiments, is to create a nonlinear mapping (i.e., lookup table) between code value and the voltage supplied to control display luminance. The latter approach is called gamma correction, and allows the display characteristic to be readily compensated with minimal computation.
Using either of the two correction methods, one needs to know the gamma curve. It is straightforward to measure the relationship between luminance and code values if one has a photometer and appropriate display software. However, in most cases, end users have no access to the required devices to perform the measurements.
It is known to use a visual matching method to determine the parameters in Equation 1, in order to provide an estimation of the nonlinear function for individual CRT displays. For example U.S. Pat. No. 5,754,222 issued May 19, 1998 to Daly et al. shows a visual matching method to determine the relationship between code value and display luminance by displaying one patch having a constant code value over a large spatial extent and another patch having two known, but different code values which are interspersed throughout the spatial pattern, to characterize a display. One of the code values is adjusted until the perceived brightness of the patches match.
When performing a characterization by matching these two patterns, this approach takes advantage of a property of human vision that allows high spatial frequency changes in light to be integrated to the average value of the two different code values in the second pattern. This integration allows the user to match the perceived brightness of the two patterns even though the pixels within these patterns have different code values.
This method has a number of inherent problems. Among these problems is that the results of the calibration may be influenced by the modulation transfer function of the display. This effect becomes stronger as the addressibility and thus the resolution of the display is increased. A second, although not independent problem, is that when the resolution of the display or the frequency of the high spatial frequency pattern is decreased, the user begins to be able to distinguish the different luminance values within the high spatial frequency pattern and has difficulty fusing the light from the higher and lower code value portions of the high spatial frequency pattern. This can result in confusion on the part of the user and reduce their ability to perform the task in an accurate manner. A third problem relates to interaction between neighboring pixels in the display device. The prior art visual assessment methods assume that the code value to luminance response of any pixel on the display is independent of the state of all other pixels on the display. It is commonly understood that this assumption is not true for CRT displays. It is also not true on passive matrix flat panel displays due to leakage current in the display.
There is a need therefore for an improved method of characterizing a video display that avoids the problems noted above.
The need is met according to the present invention by providing a method of characterizing a video display, including the steps of providing a mathematical model of the display that relates code values to luminance output of the display, the model having a parameter that characterizes the display; displaying a reference patch and a test patch simultaneously on the display, the reference patch having a reference brightness produced by one or more luminances and the test patch having an test brightness produced by rapidly switching back and forth between two luminances; observing the display while varying one of the luminances, keeping the others fixed, until the apparent brightness of the reference patch matches the apparent brightness of the test patch, and recording the code values employed to produce the luminances when a best match is made; and using the mathematical model, and the recorded code values to determine the value of the parameter that characterizes the display.
The method of the present invention has the advantage of avoiding problems in displays by displaying one region that has a constant luminance and one region whose luminance is varied between two code values over time. The present invention is most advantageous for use with a display having a fast temporal response and/or a low spatial resolution. This method takes advantage of the fact that the human eye integrates light over time, typically integrating light that is modulated at above 10 Hz in a way that is linear with increasing luminance. Since large spatial patterns are utilized, the modulation transfer function of the display does not affect the luminance response of any portion of the pattern and does not assume that the user will be able to spatially fuse the patterns that are displayed.